An ion implanter includes an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam typically is mass analyzed to eliminate undesired ion species, accelerated to a desired energy, and implanted into a target. The ion beam may be distributed over the target area by electrostatic or magnetic beam scanning, by target movement, or by a combination of beam scanning and target movement. The ion beam may be a spot beam or a ribbon beam having a long dimension and a short dimension.
Implantation of an ion species may allow a substrate to be cleaved. The species form microbubbles in the substrate material. These microbubbles are pockets of a gas or regions of an implanted species below the surface of the substrate that may be arranged to form a weakened layer or porous layer in the substrate. A later process, such as heat, fluid, chemical, or mechanical force, is then used to separate the substrate into two layers along the weakened layer or porous layer.
For example, hydrogen or helium may be used to induce cavitation, or cavities caused by microbubbles, at a predetermined depth of a substrate. A thermal treatment after the implantation may be used to aggregate the hydrogen or helium. Clustering of the hydrogen or helium will build pressure in the cavities formed by the microbubbles. A shear force, such as that found after attaching the substrate to another substrate, will cleave a thin layer off the substrate. The thickness of the thin layer depends on the depth to which the hydrogen or helium was implanted.
Ostwald ripening may occur in substrates that have microbubbles. Ostwald ripening is a thermodynamic process where larger particles grow by drawing material from smaller particles because larger particles are more stable than smaller particles. Any atoms or molecules on the surface of a particle, which may be, for example, a microbubble, are energetically less stable than the more ordered atoms or molecules in the interior of a particle. This is partly because any atom or molecule on the surface of a particle is not bonded to the maximum possible number of neighboring atoms or molecules, and, therefore, is at a higher energy state than those atoms or molecules in the interior. The unsatisfied bonds of these surface atoms or molecules give rise to surface energy. A large particle, with a greater volume-to-surface ratio, will have a lower surface energy. To lower surface energy, atoms or molecules on the surface of smaller, less stable particles will diffuse and add to the surface of the larger, more stable particles. The shrinking of smaller particles will minimize total surface area and, therefore, surface energy. Thus, smaller particles continue to shrink and larger molecules continue to grow.
FIG. 1 is a view of Ostwald ripening in a substrate. FIG. 1 is merely an illustration and is not to scale, although the x1 and x2 references will allow comparison of FIGS. 1-2. A species that forms the microbubbles 100 in the substrate 138 makes smaller microbubbles 101 and larger microbubbles 102. Due to their greater volume-to-surface ratio and lower surface energy, the larger microbubbles 102 will be more stable than the smaller microbubbles 101. To lower their surface energy, the smaller microbubbles 101 will diffuse to the larger microbubbles 102. Overall, the smaller microbubbles 101 may shrink and the larger microbubbles 102 may grow. Some of the species in the microbubbles 100 also may diffuse out of the substrate 138 or some smaller microbubbles 101 will diffuse together to form a larger microbubble 102. Ostwald ripening and diffusion will affect the substrate 138 when it is cleaved along the weakened layer or porous layer represented by line 103.
FIG. 2 is a view of the substrate of FIG. 1 after the substrate is cleaved. The substrate 138 in FIG. 1 was cleaved along the weakened layer or porous layer represented by line 103. As illustrated in FIG. 2, significant surface roughness 104 occurs due to Ostwald ripening and diffusion of the species. Due to the rough surface within a substrate when the substrate is separated into two layers along the weakened layer or porous layer, a polishing step after the substrate is cleaved may be required to make the surface of the substrate smooth enough for device manufacture. This polishing step is expensive and compromises the uniformity of the silicon on the surface of the substrate.
Implanting a species to cleave a substrate also typically has other shortcomings or inefficiencies besides the surface roughness. For example, a large dose of the implant species is usually required. For single crystal silicon, this dose may be more than 3E16 cm−2. A significant percentage of this implanted species will diffuse out of the substrate, which contributes to the high implant dose requirement. Furthermore, a complex combination of gas implants may be required to enhance the pressure in the cavities formed by the microbubbles. For example, hydrogen may be used to form cavities and helium may be used to increase pressure within these cavities. Accordingly, there is a need in the art for an improved process of substrate cleaving, and, more particularly, for a process that improves substrate cleaving using ultrasound energy.